The background of this invention started with replacing a handheld voltohmmeter for measuring the resistance of the electrolyte. Tests conducted both in the field and in the laboratory using a handheld voltohmmeter indicate different values for the ground resistances for the coated and bare pipes in the same locale. Also, due to logarithmic ohm scales, meter inaccuracy is intolerable at the higher resistivity scales. A simple solution, such as injecting a constant current into the ground and measuring the voltage drop caused by the constant current, may replace the handheld meter. Especially, if the current is one milliampere or one microampere, it is very simple to convert the voltage drop into kilohms or megohms, respectively. A bilateral constant current generator circuit was implemented and extensive tests were conducted in the laboratory along with testing in the field. The bilateral constant current generator was input to an HP 3312A function generator, and the resultant voltage waveforms were investigated. The tests established that a forced off state condition can be generated by adjusting the magnitude of the current injected into the electrolyte at a test point without interrupting the electrolysis process. Further tests were conducted to investigate the rise and fall times of the waveforms for calculating the capacitance. These tests proved the existence of the above mentioned capacitances which caused the disappearance of the waveform generated by the fullwave rectifiers used in the prior art.
In summary, this invention provides a method and an apparatus for measuring the pertinent parameters of a buried pipeline or of an offshore platform to be protected against corrosion by electrolysis without interrupting the impressed current of electrolysis or restricting the generator to any specific waveform. Several parameters and the methods of their measurements in the vicinity of a test point of a cathodically protected structure based on this invention are;
1. The equivalent voltage Ee, elsewhere named ON state Voltage, is measured at a fixed rate of sampling and digitization,
2. The equivalent resistance Re is by measured from the resultant voltage drop caused by an active signal and calculation,
3. The depletion region capacitance Cd is measured from the rise time .tau. of the resultant voltage drop caused by a constant current pulse directed into the electrolyte, and calculation from the relationship: EQU .tau.=(Cd) (Re),
4. The coating capacitance Cc is measured from the rise time of the voltage drop caused by a constant current pulse into the electrolyte across the equivalent resistance Re and calculation from the relationship: EQU t=(Cc) (Re),
5. The IR drop is measured from the voltage drop caused by the impressed current supporting electrolysis while measuring the rise time of the voltage drop immediately following a forced off condition which is then terminated,
6. The polarized potential Ep is defined by the equivalent voltage (Ee) and IR drop by calculation as represented in equation (1) above, and
7. Ion recombination time, following the IR drop measurement is from measurements of the equivalent voltage change as a function of time and calculation.